1. Field of the Invention
The present invention relates generally to electrical generators, and more particularly to electrical generators for low-frequency and time-varying rocking and rotary motions.
2. Prior Art
In recent years, particularly following the development of low-power electronics, sensors and wireless communications devices, electrical energy generators that harvest energy from the environment have seen renewed attention. The most common means of generating electrical energy is the use of magnets and coils using rotary type of generators. Generators that harvest energy from vibration that use either coils and magnets or active materials such as piezoelectric materials based devices have also been developed, particularly for low power consuming electronics. In the latter area, piezoelectric materials have been used widely to generate electrical energy from the ambient vibration. See e.g., Sodano, H. A., Inman, D. J., and Park, G., 2004, “A Review of Power Harvesting from Vibration using Piezoelectric Materials,” The Shock and Vibration Digest, Vol. 36, No. 3, 197-205.
To efficiently generate electrical energy from mechanical energy using active materials such as piezoelectric elements or by using various types of magnets and coils arrangements, the frequency of the input motion must be relatively high and not time varying. This means that if the input motion is rotary, then the rotational velocity must be relatively constant and high, preferably in the order of several thousands. On the other hand, if the motion is oscillatory, such as vibratory or rocking or the like, then the frequency of vibration or rocking must be high, preferably in the order of a few thousands when using magnet and coil type of mechanical to electrical energy conversion devices and even higher frequencies if, for example, piezoelectric based of mechanical to electrical energy conversion devices are employed.
However, in many applications, for example in platforms that rock through relatively small angles such as ships, trains or trucks; the rocking or oscillating frequency is very low and even in the order of 0.2-0.5 Hz and time varying, thereby making the operating of all currently available energy harvesting devices, i.e., mechanical energy to electrical energy conversion devices, extremely inefficient. Similarly, in rotary machinery such as windmills or turbines used to harvest tidal or ocean waves or other similar flows, the input rotary speed is relatively low and varies significantly over time, thereby making the operation of all currently available electrical energy generators highly inefficient. In fact, in most such turbo-machinery, such as windmills, to make the generation cycle efficient, gearing or other similar mechanisms have to be used to increase the output speed and in many applications to also regulate the output speed. Input speed increasing gearing and speed control mechanisms are, however, costly and significantly increase the system complexity and cost, particularly those related to maintenance and service.